1. Field of the Invention
The present invention relates to an airbag system collision history recording method of recording acceleration data on acceleration that a vehicle undergoes when the vehicle is involved in a collision in a nonvolatile memory as a collision history for an airbag system that detects the collision by using an acceleration sensor in order to secure a crew's safety.
2. Description of Related Art
FIG. 11 shows a prior art recording device intended for airbag systems, which is disclosed in Japanese patent No. 2875515, for example. In the figure, reference numeral 1 denotes a microcomputer disposed as a control unit, reference numeral 2 denotes an airbag ignition circuit that works from a signal from the microcomputer 1, reference numeral 3 denotes an acceleration sensor that detects a collision of a vehicle and sends a signal indicating the collision to the microcomputer 1, reference numeral 4 denotes a RAM connected to the microcomputer 1, and reference numeral 5 denotes a nonvolatile memory connected to the microcomputer 1.
In the prior art recording device untended for airbag systems that is constructed as previously mentioned, acceleration data on acceleration that the vehicle undergoes, which is detected by the acceleration sensor 3, are stored in the RAM 4 by way of the microcomputer 1 while the vehicle is traveling. When the RAM 4 is filled with acceleration data, old acceleration data are replaced by new acceleration data one after another. When the vehicle is involved in a collision, the recording device writes and stores all the acceleration data stored in the RAM 4 in the nonvolatile memory 5 in chronological order by way of the microcomputer 1 so that all the acceleration data can be used for analysis of the collision.
FIG. 12 is a graph showing acceleration data that are stored in the volatile memory by the prior art airbag system of FIG. 11. When a collision occurs, assuming that the airbag is inflated at a certain time tx, the prior art recording device writes and stores all acceleration data stored in the RAM 4, i.e., acceleration data on acceleration that the vehicle undergoes during the interval between the time t0 and the time tx in the nonvolatile memory 5 in chronological order (t0→tx).
As previously mentioned, in accordance with the prior art collision history storing method, at the occurrence of a collision the acceleration data stored in the RAM 4 are written into the nonvolatile memory 5 in chronological order. Therefore the prior art collision history storing method has the following problems. In general, the nonvolatile memory is a low-speed memory, and it takes much time to store data in the nonvolatile memory as compared with the time required for the microcomputer to perform processing and the duration of the occurrence of a collision. Therefore, when a failure, such as a decrease in the power supply voltage, occurs due to a break or the like caused by an impact on the airbag system at the occurrence of a collision while the microcomputer 1 is storing data in the nonvolatile memory 5, the process of storing data in the nonvolatile memory 5 is interrupted. In the example of FIG. 12, when a failure, such as a break in the airbag system, occurs at a time tα and the storing process is interrupted by the occurrence of the failure, no acceleration data on acceleration that the vehicle undergoes after that is stored in the nonvolatile memory 5 and only acceleration data on acceleration that the vehicle undergoes a time period D from the inflation time tx when the airbag is inflated to the time tα are written in the nonvolatile memory 5. However, in order to analyze the collision phenomenon that causes the inflation of the airbag, acceleration data on acceleration that the vehicle undergoes during a time period before and after the inflation time tx when the airbag is inflated, the acceleration data showing a strong correlation with the inflation of the airbag, are useful for the analysis of the collision, and the acceleration data on the time period D that are old acceleration data show a weak correlation with the inflation of the airbag.